A reputed mobster carried out a hit on an off-duty New York Police Department officer who made the mistake of marrying the ex-wife of a vengeful acting mob boss, a prosecutor said Monday in opening statements at a Brooklyn trial.

The slaying of Officer Ralph Dols in the summer of 1997 is one of six gangland murders from the 1990s that can be traced to Thomas "Tommy Guns" Gioeli, Assistant U.S. Attorney Cristina Posa said.

The acting boss of the Colombo organized crime family "wanted Ralph Dols dead and he turned to that man," Posa said, pointing to Gioeli in the courtroom, "to get it done."

The six homicides had gone cold for a decade or more until the government cultivated a new crop of mob turncoats who agreed to testify against Gioeli and another alleged Colombo crime family member who's standing trial with him.

Some of the cooperators have pleaded guilty to avoid the death penalty, defense attorney Carl Herman said Monday. He accused the government of "offering deals to criminals and murderers."

Herman also claimed there was no physical evidence linking the 59-year-old Gioeli to any of the killings.

"You won't see any fingerprints or DNA or ballistics," he told the jury. "Don't wait for that _ it's not coming."

Dols, 28, was ambushed around midnight as he arrived home from a shift as a uniformed housing police officer. While parking his car, a man jumped out of a dark-colored Chevrolet, fired seven shots, and fled.

The killing touched off an intense, wide-ranging investigation involving federal and local authorities. It also drew attention to the officer's wife and her alleged links to the Mafia through three other men from her past: a brother and reputed Colombo soldier who was convicted of murder in 1981, a previous husband found shot to death in 1987 in an apparent mob hit, and Joel Cacace _ the acting Colombo boss and her husband before Dols.

The police officer "had the bad sense to marry a mobster's wife," Herman said. However, Gioeli "wasn't present when Ralph Dols was killed. He never conveyed any order," he added.

Cacace, who's also charged in the Dols case, is expected to be tried separately.

Dol's widow, Kimberly Kennaugh, did not attend the opening statements. In an interview with the Daily News published Sunday, she said she hopes the trial will clear her name and bring justice for her dead husband.

"I've been attacked as if I pulled the gun," she said. "I want closure for Ralph's family. I want closure for him. ... I want closure too."